Progressive failure modelling and ductility demand of steel beam-to-column connections in fire

Abstract

A numerical procedure has been developed to model the sequences of failure which can occur within steel beam-to-column connections under fire conditions. In this procedure two recent developments, a static-dynamic solution process and a general component-based connection element, have been combined within the software Vulcan in order to track the sequence of local failures of the connections which lead to structural progressive collapse in fire. In particular the procedure developed can be used to investigate the structural behaviour in fire, particularly the ductility and fracture of different parts of the steel-to-steel connections, and the influence of the connections on the progressive collapse resistance of steel frames in fire. In the component-based connection model, a connection is represented as an assembly of "bolt-rows" composed of components representing different zones of mechanical behaviour whose stiffness, strength, ductility and fracture under changing temperatures can be adequately represented for global modelling. The potential numerical instabilities induced by fractures of individual connection's components can be overcome by the use of alternate static and dynamic analyses. The transfer of data between the static and dynamic analyses allows a seamless alternation between these two procedures to take place. Accuracy and stability of the calculations can be ensured in the dynamic phase, provided that the time steps are set sufficiently small. This procedure has the capacity of tracking the sequence of local failures (fractures of connection components, detachment and motion of disengaging beams, etc.) which lead to final collapse. Following an illustrative case study of a two-bay by two-storey frame, the effect of ductility of connections on the collapse resistance of steel frames in fire is demonstrated in two case studies of a generic multi-storey frame. It is shown that the analytical process is an effective tool in tackling the numerical problems associated with the complex structural interactions and discontinuous failures which can affect a steel or composite frame in fire, potentially leading to progressive collapse. It can be seen that both tensile and compressive ductility in the connections make a contribution to the fire resistance of the beams. Preventing the detachment of steel beams in fire can be achieved by inducing greater ductility into their connections. Combined with appropriate component-based connection models, this procedure can be adopted in performance-based fire-resistant design to assess the ductility requirements of steel connections.